1) Field of the Invention
This invention relates to integrated circuits and semiconductor devices. It relates to methods of filling contact holes and contact structures made by those methods and more specifically, this invention relates to a method and a structure for forming sidewall and bottom silicide in a contact hole.
2) Description of the Prior Art
In the fabrication of semiconductor devices, electrical contacts are typically made by opening contact holes in a dielectric isolation layer to expose regions of the semiconductor device to which electrical contact must be made, and depositing a layer of metal over each contact hole. The layer of metal extends from the bottom of the hole, up the sidewalls of the hole and over the dielectric isolation layer to another contact hole or to a contact pad overlying the dielectric isolation layer.
In a conventional process, the layer of metal, typically Al--Si is deposited by DC magnetron evaporation or electron beam evaporation. Such physical deposition techniques provide good coverage for horizontal surfaces, but do not always cover vertical surfaces which can cause problems in deep contact holes.
Where an electrical via for interconnection purposes has a high aspect ratio, or ratio of depth to diameter of the via, traditional metal sputtering, such as TiW/Al/TiW, has become a limiting process in submicron designs because of poor metal step coverage, poor electro-migration resistance, and higher than acceptable electrical contact resistance.
Selective chemical vapor deposition (CVD) of tungsten in the vias and on electrical contact regions has been used to overcome some of these problems. In the conventional selective CVD process, a refractory metal, such as tungsten is deposited only on exposed silicon at the bottoms of the contact holes. The selective CVD may be continued until the tungsten fills the contact holes. Because the tungsten is selectively deposited in the contact holes, etching of unwanted tungsten is required when one or more of the contact holes is overfilled. Overfilling of a contact hole results in an excess metal formation above the surface which is sometimes called a "nail's head" because of its shape. Overfilling is common in devices where the contact holes have different depths. In addition, the selective chemical vapor deposition (CVD) of tungsten has other problems that must be overcome. First, trapping of a polycrystalline insulator, such as a titanium trifluoride, TiF.sub.3, at the tungsten/titanium silicide (W/TiS.sub.x) interface adjacent to a via or an electrical contact will induce high contact resistance. Second, selectivity loss of an oxide surface in tungsten deposition may induce severe defects and reduce circuit yield. During CVD of tungsten (W), an exposed adjacent oxide surface may become contaminated by a photoresist patterning process so the, contaminated oxide behaves as a tungsten nucleation layer that will include severe defects. Third, the tungsten plug must be formed to completely fill contact hole having a narrow diameter and large depth.
U.S. Pat. No. 4,898,841 to Ho teaches a method of filling contact holes whereby a tungsten silicide layer is formed on the side-walls of a contact hole.
U.S. Pat. No. 5,286,675 to Chen et. al, teaches a method of forming a contact using a blanket tungsten etch back step and a disposable (etch back the SOG layer which underlies the blanket W layer) SOG layer.
U.S. Pat. No. 4,963,511 to Smith, teaches a method of forming a tungsten plug whereby a tungsten non-selective layer is formed on a contact hole sidewalls thus inhibiting tungsten from forming on the sidewalls.
U.S. Pat. No. 5,219,789 to Adan discloses a method of forming a contact whereby a SOG layer is used to mask a TiW layer which coats the surface of a contact hole.
U.S. Pat. No. 5,187,120 to Wang discloses a contact formation method whereby a first TiN layer is formed on the bottom of a contact hole in a dielectric layer to promote the nucleation of tungsten. A second layer which resists nucleation of tungsten is formed on the surface of the dielectric layer.
While these methods improve the reliability and manufacturability of tungsten electrical contacts, further improvement is desirable.